Interest in the use of ion-exchanged glass waveguides for passive integrated optics has increased considerably recently. Typical examples of such components and devices are single-mode 1/N power dividers and wavelength selective directional couplers. Since the operation of optical glass waveguides is purely passive, and does not allow adjustment after production, accurate control of production parameters is needed to obtain the desired waveguide reproducibility. This is especially important in the case of directional couplers, in which an accurate control of the propagation constant difference between symmetrical and asymmetrical modes is necessary. To produce optical fiber compatible waveguides by an ion exchange technique, two-step processes are generally used. In these processes, waveguides are formed by the exchange of the original ions in the glass (typically sodium ions Na.sup.+) to ions increasing the refractive index (such as K.sup.+, Ag.sup.+, Cs.sup.+, Rb.sup.+, Li.sup.+ or Tl.sup.+ ions) through a narrow opening in the ion exchange mask, and by using salt melts or a silver film as an ion source. In the second step, thermal treatment or ion exchange in an NaNO.sub. 3 melt modifies the refractive index profile of the waveguide to obtain better coupling to optical fiber. As the diffusion during the second step increases the width of waveguides, narrow (.apprxeq.2-4 .mu.m) mask openings must be used in the first step. The number of exchanged ions is very sensitive to the mask opening width, wherefore there is a marked need of a new production method which allows a wider range of variation for the mask opening width. As to the basic principles of the ion exchange technique, the following article is referred to:
Ion-Exchanged Glass Waveguides: A Review, R. V. Ramaswamy, Journal of Lightwave Technology, Vol. 6, No. 6, Jun. 1988, p. 984.